EFFICIENCY IMPROVEMENT IN PULVERIZED COAL BASED POWER
Pradip Kumar Mandal
Head, Coal -Combustion & X-Ray Division, NTPC Limited, Research & Development Centre, 8A, Sector-24,
Noida, U.P., 201301, India, Mobile No. : 9868391472, Fax: 2410311, e-mail: firstname.lastname@example.org
There are many technologies available to generate electricity but pulverized coal-fired boiler systems will continue
to be major contributors of electricity in the future world over. However, it needs total up gradation of present
pulverized coal-fired systems (thermal cycle efficiencies ranging from 32 to 35%, higher heating value) by new third
generation technologies (efficiencies in excess of 42% or even more). To attain the improved efficiency the
technology up gradation in the field of cycle efficiency, advanced combustion system/ furnace design, variable and
dual pressure operation, thermal hydraulic design, low level heat recovery, boiler materials, advanced SO x emission
control, etc. have to be undertaken.
This paper highlights some of today’s design improvements which target reduced emissions and expanded
operability, and explores some of the boiler design implications for the ultra-supercritical conditions needed to
achieve the high cycle efficiencies(~55% ) for the future.
Keywords: Advanced combustion system, sub critical, supercritical & ultra supercritical boilers, heat rate,
thermal efficiency, cycle efficiency, thermal hydraulic design, variable & dual pressure operation, SO x emission
To assure the electricity supply in the future there is a strong need for coal-fired power plants. Today’s energy
sources are composed of fossil fuels, water power, and nuclear energy. To lesser extent, wood, solar, wind, tidal,
geothermal and chemical resources are used but huge demand of electricity world over can only meet by coal based
power stations [1-3].
Advanced coal technologies aim to increase the amount of electrical energy extracted from a unit of coal. The key,
here, is higher overall conversion efficiency, which will reduce the emissions of CO 2. Technologies in this category
i) Various advanced pulverized coal (PC) combustion technologies (modified sub-critical, supercritical and ultra-
ii) Fluidized bed combustion (FBC) technologies (circulating and Pressurised); and
iii) Coal gasification combined cycle (CGCC) technologies. Efficiencies range from 40 – 50 % compared 33 to 35 %
for a conventional PC unit.
2. Present Technologies (sub-critical)
The basic design features of present pf boilers (efficiencies ranging from 32 to 35%, Figure 1) include a completely
water-cooled furnace, balanced draft design, horizontal and vertical convection passes to minimize initial capital
cost and wet or / dry ash removal. Natural circulation provides the furnace wall cooling. Superheat and reheat
surfaces are of the vertical pendant and horizontal designs. Superheat temperature is controlled by a spray
attemperator and reheat temperature is controlled by gas proportioning dampers at the outlet of the reheater and
Figure 1 Conventional pulverized coal based power plant
3. Efficiency improvement in conventional pulverized coal plants
3.1 Current Heat rates
Unit efficiency( heat rate) is a function of unit design, size, capacity factor, the fuel fired , maintenance condition of
the unit, and operating and ambient conditions (cool water temperature).Existing pulverized coal boilers operating
today is mainly sub critical (major) or supercritical (minor) steam cycles. A supercritical steam cycle normally
operates above the water critical temperature (705 ºF) and critical pressure (3210psi) where water can exist only in
the gaseous phase. Sub critical systems have achieved thermal efficiency of 33 to 34 %.Super critical systems
achieve thermal efficiency 3 – 5 % higher than sub critical systems. Table 1 summaries some of the potential actions
that could be taken to improve plant efficiencies.
Table1. Measures that may improve the efficiency of coal – fired power plants
Action Efficiency Improvement (%)
Restore plant to design conditions
Minimize boiler tramp air 0.42
Reinstate any feed heaters out of service 0.46 – 1.97
Refurbish feed heaters 0.84
Reduce steam leaks 1.1
Reduce turbine gland leakage 0.84
Changes to operational settings
Low excess air operation 1.22
Improved combustion control 0.84
Extra air heater surface in the boiler 2.1
Install new high efficiency turbine blades 0.98
Install variable speed drives 1.97
Install on-line condenser cleaning system 0.84
Install new cooling tower film pack 1.97
Install intermittent energisation to ESPs 0.32
In addition, to have better performance in present system improvements have to be incorporated in a number of
b) Low NOx Combustion systems,
c) Start-up / low Load Systems,
d) Super heater Headers,
e) Post Combustion Systems,
f) Variable Frequency / variable Speed Fans, and
3.2 Advanced Pulverized Coal –fired Boiler systems (Supercritical technologies)
World over many developmental work are going on for advanced pulverized coal – fired systems to meet future
power requirements in a cost effective and environmentally acceptable manner. The main efforts are:
i) Increase thermal efficiency,
ii) Improved environmental performance,
iii) Increased availability,
iv) Control of initial capital cost, and
v) Reduced overall cost of electricity
The main objective of the advanced systems is mainly:
a) Thermal efficiency greater than 42 %( HHV). Finally it may be as high as 50-55 %( HHV)
b) Emission levels (mg/Nm3): SO2, < 120 – 250; NOx , < 120; TDS, < 20 – 40;
c) Increased availability, reliability and fuel flexibility.
In the advanced systems the following aspects are very important:
i) Cycle efficiency,
ii) Advanced combustion system/furnace design
iii) Variable and dual pressure operation
iv) Spiral and vertical furnace circuitry
v) Thermal – hydraulic design
vi) Boiler materials
vii)Low level heat recovery, and vii) advanced SO2emmission control
i) Cycle efficiency,
The net efficiency of a sub critical boiler was in the range of 33 to 34%HHV. Recent plant design have incorporated
improvements in a number of areas to increase the efficiency of this cycle to between 35% and 37 % by optimizing
feed water heating, , lower turbine exhaust pressure through lower cooling water temperatures , improved steam
turbine efficiency, etc. To achieve this it is necessary to increase the steam pressure and temperatures (called ultra
supercritical steam conditions [3, 4-5].
166 bar/538º C/538 ºC (drum: single reheat) , Efficiency, 33 – 34 %
241 bar/538º C /538 ºC (once – through: single reheat), Efficiency, 42% widely in use
276 - 310 bar/538º C /552º C /556 ºC (once – through: double reheat), Efficiency > 42%Considered feasible
~ 414 bar & temperature, 600 – 700º C , Efficiency ,50 - 55% , Research stage .
The plant efficiency with steam temperatures is given in Figure 2.
Figure 2. Effect of increasing steam temperature and pressure on cycle efficiency
ii) Advanced combustion system
Boiler furnace geometry and dimensions are intimately connected with the combustion system and fuel(s) to be
employed. Furnace cross-sectional area must accommodate the flame shape plus burner arrangement/spacing to
iii) Variable and dual pressure operation
In variable boiler pressure operation, the boiler operation pressure is changed to meet the turbine load requirements,
improving the low load heat rate.
iv) Thermal hydraulic design
Special geometry ribbed bore tubes are required to be developed to maintain adequate tube wall cooling under broad
range condition( supercritical , water acts as single phase fluid and under sub critical pressure where two- phase
steam –water flow occurs).
v) Boiler materials
It is a most challenging issue for development of high temperature, high strength corrosion resistant materials for
boiler tubes of improved temperature and pressure conditions. In order to realize a 700º C ultra supercritical power
plant, extensive materials development is necessary including the use of Ni-based super alloys in the most severely
exposed components. The effect of temperature on various metals is given in Figure 3.
Figure 3. Future materials for thick walled components
vi) Change of coal characteristics
Change of coal quality (poor grade coal) by washing, blending and additive dosing may improve the efficiency of pf
4.1 Case Studies
Dadri power station of NTPC started using washed coal since 1997. Quality of washed coal is more consistent with
4 – 5 % reduction in ash content and total elimination of stones. Besides stable boiler operation this has helped in
increasing life of crusher hammer, coal mill bull ring segments and grinding rolls, coal nozzles assembly, boiler
pressure parts , improved ESP performance, and thus overall stable plant operation, improved plant availability,
plant load factor, efficiency / heat rate , etc.
Coal blending is a process in which two or more single coals are mixed intimately in required ratio to obtain a
product of desired properties better than parent coal. Worldwide coals are blended to smooth out extreme and
harmful properties or to improve combustion characteristics resulting efficiency improvement.
4.3 Additive dosing to better pulverization to improve the efficiency
Coal fineness is drastically affecting the efficiency of pc burning. Some moisture and clay rich coals are difficult to
burn but additive dosing may improve the situation (Figure 4) and finally effective combustion and increase of
Figure 4. Fineness improvement by additive dosing in coal pulverizing mill
5. Present status of advanced technologies
Research activities and field - trials are currently underway world over to develop even more efficient and clean
pulverized coal fired systems which will compete effectively with other sources of electrical power for many years
The overall technical objective of the advanced technologies is a phased development and demonstration of an
economically viable, pulverized coal fired power plant technology with a net efficiency of more than 50%, the
potential for which has already been established during the first phase of the study in some of the counties.
An advanced super critical water/steam cycle will boost maximum steam temperatures from the standard 540 – 560º
C range to the 700 – 720º C range and main steam pressure from the present 250 bar range to the 350 – 375 bar
The phase 1 results indicate net efficiencies in the range of 50 – 51% for a power plant with a single reheat cycle
cooled by a wet cooling tower and 53 – 54 % for a double reheat cycle cooled by sea water. Plant sizes studied are
400MW and 900 MW.
Many countries world over had already taken efforts to install super critical technologies to improve efficiencies in
pulverized coal fired boiler. In some European countries after 40 years of experiences on supercritical boilers it was
found an impressive efficiency improvements of coal fired power plants from a mere 30 % in 1950 to 45 % in 1992
and further to 47 % in 2001.Future improvements up to 52% (max, for steel based seawater cooled plants) and a new
generation of advanced plants based on super alloys may start at around 2010 with efficiency to 55%.
Now the time has come to introduce the advanced super critical technologies in those regions where the need of
power is growing most rapidly. In particular, the fast growing economies of Asia can demonstrate high growth rates
for electric power, and the coal based electricity generation in China and India will rise by 4 – 5 %. That means, if
the annual generating efficiency in India and China (Giant Asian countries) could be improved from the present
figures in the range of 30% into the range of 45% by applying the existing advanced super critical technologies, the
Chinese and Indian coal use in 2015 could be reduced by some 30%. Some of the high efficiencies of advanced pulverized coal
based power plants is given in Table 2.
Table 2.Some of the high efficiencies of conventional pulverized coal based power plants
Plant year Size (MW) Steam temperature (F) Design thermal efficiency (%) < HHV
Plant-a 1992 550 1004/1040 41.1
plant-b 1994 550 1004/1040 42
plant-c 1992 400 1036/1040 43.2
plant-d 1998 400 1076/1076/1076 44.9
(Double reheat cycle)
plant-e 1998 440 1076/1112 43.6
plant-e 2002 750 1067/1103 44.2
plant-f 1998 400 1076/1076/1076 47.0
6. Other Possibilities for efficiency improvement
6.1Combined Cycle Operations at Coal – Fired Power Plants:
Coal – fired power plants have historically been limited to the simple cycle method. However, recent technological
developments have led to the capability of powering combined – cycle generators. Two new technologies __
Pressurised Fluid Bed Combustion and Integrated Gasification Combined Cycle (IGCC) have dramatically
improved efficiencies or heat rates as compared to conventional pulverized coal – fired facilities [4 -5]. A 350 MW
combustor is under construction in Japan and expected efficiency is 41%whch has further potential to reach 43% in
future plants. One study notes that the efficiency of IGCC plants is expected to be around 42 % and there is potential
to achieve 49 % when higher efficiency gas turbines become available.
Electricity requirement increases many fold every year world over. There are many routes to generate electricity.
Today’s energy sources are composed of fossil fuels, water power, and nuclear energy. To lesser extent, wood, solar,
wind, tidal, geothermal and chemical resources are used but huge demand of electricity world over can only be met
by coal based power stations. Fossil fuels are non renewable energy sources and depleting rapidly. So it is expedient
to improve the efficiency of pulverized coal based thermal power stations by advanced technologies. Technologies
in this category include: i) Various advanced pulverized coal (PC) combustion technologies (modified sub-critical,
supercritical and ultra-supercritical); ii) Fluidized bed combustion (FBC) technologies (circulating and Pressurised);
and iii) Coal gasification combined cycle (CGCC) technologies. Efficiencies range from 40 – 55 % compared 33 to
35 % for a conventional PC unit. Now technologies are available or going to be available with efficiencies as high as
55% as against conventional, 30 – 34 %. This will reduced the use of coal to the tune of 30 % with huge reduction
of CO2 emission and will sustain human friendly environment further for many years to come
The technical realization of a 700ºC steam power plant depends on a successful development and qualification of
advanced ferritic, austenitic and Ni- based alloys. With respect to austenitic and Ni-based alloys efficiency of the
order of 55 % can be achieved.
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